1. Field of the Invention
The present invention relates to a heat-sensitive recording material and a heat-sensitive recording process, and more particularly, to a heat-sensitive recording material and a heat-sensitive recording process of high image quality that is suitable for a medical-use recording medium.
2. Description of the Related Art
A heat-sensitive recording process has such advantages that (1) development is not necessary, (2) in the case where a support is paper, the material is similar to ordinary paper, (3) easy handling, (4) the coloring density is high, (5) a recording apparatus is convenient and has high reliability at low cost, (6) no noise during recording, and (7) no maintenance. Therefore, it has developed in various fields in recent years, and has spread, for example, in the field of facsimile machines and printers and the field of labels, such as POS systems.
As heat-sensitive recording materials used in the heat-sensitive recording process, materials utilizing a reaction between an electron donative colorless dye and an electron acceptive compound and materials utilizing a reaction between a diazo compound and a coupler have been known.
Against this background, projection of images and the like with overhead projectors, direct observation of images and the like on light tables, and the like are in demand. In order to meet this demand, and in response to the spread of multi-color systems in recent years, the development of transparent heat-sensitive recording materials that can be directly recorded by thermal heads is desired.
Accordingly, a heat-sensitive recording material has been proposed that is produced in such a manner that a substantially colorless coloring component A and a substantially colorless coloring component B that colors through a reaction with the coloring component A are dispersed in a binder in fine particle form or, in alternative, one of the components A and B is formed into microcapsules, and the other is formed into an emulsion, so as to form a heat-sensitive recording layer which is provided on a transparent support, such as a synthetic polymer film.
In this transparent heat-sensitive recording material, although the transparency of the material itself is good, it has problems in that sticking and noise are liable to occur when printing in a heat-sensitive recording apparatus, such as a heat-sensitive printer. Particularly, in cases where the transparent heat-sensitive recording material is used for medical applications, the heat energy applied by a thermal heat is large, due to a high transmission density demanded, and thus problems of sticking, noise on printing and wearing of a thermal head become serious. In order to prevent sticking and noise, there have been attempts to provide a protective layer containing a pigment and a binder as main components on the heat-sensitive recording layer, and to further add various lubricating agents in the protective layer. However, this is not preferred in medical applications since the lubricating agent attaches to the head as contamination and causes unevenness in image density.
Because high energy recording with a high black ratio is conducted in images of medical applications, the thermal head is worn, which changes a thermal conduction property from a head-heating element, whereby unevenness in density occurs. It has been proposed to provide a layer containing chemically stable carbon having a high hardness as a main component on the surface of the head to prevent wearing of the thermal head. However, the carbon layer has such characteristics that a surface energy is lower than the surface layer of a conventional thermal head (such as SiN and SiC), which increases a friction coefficient with the protective layer of the heat-sensitive recording material, and increases the problems of sticking and noise.
An object of the present invention is to solve the problems associated with the conventional art, and thus to provide a heat-sensitive recording material that is excellent in a head-matching property.
Another object of the present invention is to provide a heat-sensitive recording material of high image quality in which energy dependency of a friction coefficient between a head and the heat-sensitive recording material is suppressed and a density step caused by the printing energy is decreased.
The present invention relates to, in one aspect, a heat-sensitive recording material having: (a) a support; and (b) a plurality of layers formed on the support, including at least a heat-sensitive recording layer and a protective layer formed on the heat-sensitive recording layer, the protective layer including at least one compound represented by one of the following structural formulae (1), (2) and (3) or a water-soluble polymer having a repeating unit represented by the following structural formula (4): 
in which, in the structural formulae (1), (2) and (3), X represents H or CH2OH; R1, R2, R3 and R4 each represents a saturated or unsaturated alkyl group having from 8 to 24 carbon atoms, which alkyl group is optionally branched and optionally has a hydroxyl group; R3 and R4 may be the same and may be different; and L represents the following structural formula (5): 
in which n+m is an integer from 0 to 8, and, in the structural formula (4), Y represents H, xe2x80x94SO3A or xe2x80x94COOA, and A represents Na, K, NH4 or NH(C2H4OH)3.
Preferably, in the heat-sensitive recording material, the at least one compound represented by one of the structural formulae (1), (2) and (3) is contained in the protective layer in an amount of from 0.5 to 10% by weight based on total dry coating amount of the protective layer.
In another aspect of the present invention, in the heat-sensitive recording material, the at least one compound represented by one of the structural formulae (1), (2) and (3) is selected from the group consisting of stearic amide, ethylene bisstearoamide, methylol stearoamide, lauric amide, ethylene bislaurilamide, myristic amide, palmitic amide and behenic amide.
Preferably, in the heat-sensitive recording material, the protective layer contains stearic amide in an amount of from 0.5 to 10% by weight based on total dry coating amount of the protective layer.
In a further aspect of the present invention, in the heat-sensitive recording material, the support and the heat-sensitive recording layer are substantially transparent.
In yet another aspect of the present invention, a thermal head is placed in contact with the heat-sensitive recording material during image recording, and a difference in transportation torque when applying a minimum amount of heat for causing coloring in the heat-sensitive recording material and when applying an amount of heat for causing a color transmission density of approximately 3.0 is no more than 2 Kgxc2x7cm.
In a yet further aspect of the present invention, in the heat-sensitive recording material, the protective layer includes the water-soluble polymer having a repeating unit represented by the structural formula (4).
In still another aspect of the present invention, in the heat-sensitive recording material as claimed in claim 7, the water-soluble polymer includes at least a water-soluble polymer represented by one of the following structural formulae (6), (7), (8) and (9): 
in which Y represents xe2x80x94SO3A or xe2x80x94COOA, and A represents Na, K, NH4 or NH(C2H4OH)3; m represents an integer of at least 10; n represents a number from 0.1 to 0.9, l represents a number from 0.9 to 0.1, and n+l is 1.0; r represents a number from 0.1 to 0.9, s represents a number from 0.9 to 0.1, and r+s is 1.0; t represents a number from 0.1 to 0.9, u represents a number from 0.1 to 0.9, v represents a number from 0.1 to 0.9, and t+u+v is 1.0; R represents an alkyl group having 2 or more carbon atoms; and Z represents Na, K, NH4 or NH(C2H4OH)3.
Preferably, the protective layer contains the water-soluble polymer in an amount of from 1 to 10% by weight based on total dry coating amount of the protective layer.
A still further aspect of the present invention is a heat-sensitive recording process featuring the steps of: (a) preparing the heat-sensitive recording material; and (b) subjecting the heat-sensitive recording material to heat using a thermal head which has an uppermost layer having a carbon content of at least 90%.